Optical transceiver devices are well known in the art and are often found in applications where a bi-directional data stream is employed for the transmission of both video, in one direction, and data, in two directions, over a single mode fiber optic cable. A 1300 nm emitter and a 1300 nm receiver are generally used for data communication and a 1550 nm receiver for video information.
Typically, these devices have been produced using conventional bulk optical elements such as lenses, WDM filters, and beam splitters, built into the device, to separate the analog and digital transmission wavelengths and to direct the signals back and forth from the emitter and receivers to the input optical fiber carrying the transmission signal. Recently, a new generation of devices has been demonstrated using planar light circuit (PLC) technology.
A major problem with conventional bulk optic transceiver devices is scattering of light from the edges and surfaces of the various components, lenses, WDM filters, and beam splitters, leading to cross-talk, i.e. light from the external 1300 nm emitter leaking into the photodetector receivers, and poor wavelength isolation due to insufficient blockage of selective light by the dielectric filters. In addition, in order to maintain a high level of wavelength isolation i.e. >25 dB, for example, the overall light polarization requires that incident light be close to a normal incidence angle at the surface of the dielectric filters. Any incident light at a different angle is leaked through the filter.
To circumvent the cross-talk and high wavelength isolation problems associated with bulk optic transceiver devices, prior art Japan Patent Office Publication number 11-068705 discloses a two-way optical transmission and reception module based on a PLC chip, in which signals at first and second (1550 nanometer and 1310 nanometer) wavelengths are separated from an input light beam using a WDM filter assembly placed in a slot in the PLC chip. The dielectric multi-layer filter, inserted into the slot, is positioned at the tee of a branching waveguide structure to obtain the aforementioned purpose. Japan Patent Office Publication number 11-248977 also discloses transceiver devices with filters inserted into waveguide structures.
The use of waveguide structures significantly reduces light scattering because of the light confinement characteristics of waveguides, resulting in essentially no cross-talk of light between the emitter and the receivers of the device. In addition, the combination of waveguide structures and dielectric filters inserted in the waveguide enhances the wavelength isolation properties of the device.
A major manufacturing problem with prior art PLC based transceiver apparatus is the required filter thickness tolerance, on the order of micrometers, necessary for positioning the dielectric filter in a slot created directly in the optical path of the waveguides of the device. In order for the transmitted light signal to cross the filter, from one waveguide into another, without a significant loss off light at the coupling interface, the thickness of the slot, typically 20 micrometers, needs to be carefully controlled, as small deviations in thickness can result in significant power attenuation. Similarly, the thickness of the filter, typically 14 micrometers, needs to be carefully controlled in order to not exceed the thickness requirement. As such, the handling of thin filters creates manufacturing difficulties.
In addition, the fabrication of a waveguide channel trench designed to receive the filter requires a relatively large surface area chip, increasing the size and cost of the apparatus.